The invention relates to a method for operating an internal combustion engine with an emission control system with at least one catalytic and/or filter-effective emission control component.
Internal combustion engines having an emission control system with a catalytic and/or filter-effective emission control component, generally require that it reaches its operating temperature as quickly as possible. It is, for example, suggested in German Patent document DE 197 49 400 A1 to determine an efficiency for an SCR catalyst and to change certain internal combustion engine operation variables when it falls below a predetermined value in such a manner that the exhaust gas temperature is increased. However, heating of the emission control system, particularly in connection with a cold start or warm-up of the internal combustion engine, can result in an undesired smoke emission, particularly a white smoke emission.
Exemplary embodiments of the present invention provide a method for operating an internal combustion engine with an emission control system with a catalytic and/or filter-effective emission control component, where an emission of smoke to the environment, particularly of white smoke and particularly in connection with a cold start or a warm-up of the internal combustion engine, is reliably limited to tolerable low values.
With the method for operating an internal combustion engine having an emission control system according to the invention with at least one catalytic and/or filter-effective emission control component, an estimate of an HC storage amount of hydrocarbons (HC) stored in one or several of the at least one emission control component takes place and a cold start engine operation method with predefined internal combustion engine operation variables is activated, if the estimate results in the HC storage amount exceeding a predefined HC storage amount limit value. HC are hereby meant to be hydrocarbons, wherein an amount or concentration determination for better comparability can be standardized to a certain hydrocarbon compound, as for example methane or hexane. Emission control components in the sense of the invention are meant to be exhaust gas catalysts such as oxidation catalysts, nitrogen oxide storage catalysts, SCR catalysts or catalytically coated or uncoated particle filters.
The specific cold start engine operation method provided for the case of an HC storage amount exceeding the HC storage amount limit value is preferably not used or is used at the most in a changed form, if, or as long as it is below the HC storage amount limit value. An excess fuel consumption can be avoided thereby, which typically adjusts with the provided specific cold start engine operation method, if this is not necessary from the view of the undesired smoke or white smoke emission.
The inventors have recognized that an undesirably high smoke or white smoke emission is caused in the first instance by a too large amount of HC adsorbed in one or several emission control components of the emission control system. It was noticed surprisingly that emission control components with a zeolite coating have to be viewed as particularly critical in this regard. If the emission control system has an oxidation catalyst and/or a particle filter with a zeolite coating in addition to a zeolite SCR catalyst, the SCR catalyst typically has the higher HC storage capacity. A smoke emission due to desorption of previously stored HC can nonetheless be dominated by the oxidation catalyst and/or particle filter.
By means of the estimate of the HC storage amount carried out according to the invention, the risk of an HC desorption or smoke emission due to a heating of an emission control component with an ability for storing HC, particularly a correspondingly designed SCR catalyst, oxidation catalyst and/or particle filter in connection with a cold start or a warm-up of the internal combustion engine can also be estimated. If the HC storage amount in the emission control system exceeds the critical HC storage amount limit value altogether or in an emission control component dominant with regard to a smoke emission, the cold start engine operation method is activated. A heating of the emission control system or a dominant emission control component with regard to an HC adsorption takes place so early on the one hand that the smoke emission remains limited to predefined values. On the other hand, the cold start engine operation method permits adjustment of a heating speed of the emission control system or a dominant emission control component with regard to an HC adsorption in a defined manner so that the smoke emission remains limited to predefined values.
In an arrangement of the invention, values of internal combustion operating variables provided for an activated cold start engine operation method in such a manner that the at least one emission control component is heated by the exhaust gases discharged by the internal combustion engine in such a manner that a rate resulting due to the heating falls below a predefined desorption rate value. It was noticed that a fast heating of an emission control component charged with a stored HC can result in a quickly increasing desorption of HC, that is, a high desorption rate and thus a high smoke emission. This is particularly the case if a desorption temperature region of typically +50° C. to +250° C. is reached or passed through during the heating. Depending on the size of the HC storage amount, a greater or less high maximum concentration of HC in the exhaust gas is emitted to the environment. It is thereby particularly preferred if a predefined heating gradient maximum value for a heating gradient of the emission control system is fallen below in regard of a dominant emission control component smoke-causing HC desorption in a further arrangement of the invention. By means of the values of internal combustion engine variables adjusted in dependence on the HC storage amount and/or the temperature particularly on the emission control component dominant with regard to a smoke-causing HC desorption, the heating gradient and thus the desorption rate value or the HC maximum concentration can be influenced in a defined manner and preset or predefined limit values can be fallen below in a reliable manner. For example, a comparatively slow heating of less than about 10° C. per min, particularly in a temperature interval of −30° C. to +230° C., a soft HC desorption can be achieved, where a critical smoke emission peak is avoided. The adjustment of a low heating gradient is particularly advantageous, if an emission control component dominant with regard to a smoke-causing HC storage, as e.g. a zeolite oxidation catalyst, has a temperature just below or within the desorption temperature region. It is particularly advantageous, starting with low temperatures of the emission control component, that is, less than 0° C., particularly less than minus 20° C., to initially adjust a high heating gradient of about 20° C./min or more in a first heating step. The exposition duration of the emission control component in the temperature regime critical for an HC store is thereby shortened, a further store is largely avoided and the HC storage amount is thus limited. If a temperature just below the desorption temperature region, that is, below about 10° C. is reached, a low heating gradient below the heating gradient maximum value is adjusted. It is thereby advantageous to adjust the heating gradient in dependence on temperature, particularly with increasing temperature.
In a further arrangement of the invention, the estimate of the HC storage amount is based on an operation duration of the internal combustion engine with a temperature for a dominant emission control component dominant with regard to a smoke-causing HC desorption falling below a predefined temperature, particularly a first threshold temperature. Longer weak load operating times at low temperatures have proved especially critical. If, for example, the internal combustion engine is operated for a longer time in the idle run below a material-specific threshold temperature of typically about 30° C., emitted HC enrich increasingly in a respective emission control component enabled for HC adsorption. With multiple successive cold start and/or warm-up processes, where particularly this emission control component continuously has temperatures, where a storing of HC can take place, the respectively stored HC amounts accumulate. With a following, particularly fast heating due to an increasing internal combustion engine load, an undesired high HC desorption and smoke emission can result. According to the invention, this is met in that the HC storage amount over the operating time with HC storage is estimated at least integrating for an emission control component dominant for a smoke-causing HC desorption. If several emission control components with an HC storage capacity are present, the combined HC amount stored in the emission control system is preferably determined by estimation. If only one emission control component dominant with regard to an HC storage is present, it can be sufficient if the HC storage amount is only determined with this component. Due to the estimate of the HC storage amount provided according to the invention, the cold start engine operation method can already be activated before reaching a critical integral HC storage amount and a defined heating of the emission control system can be achieved. For estimating the HC storage amount, one preferably refers back to stored emission characteristic fields of the internal combustion engine and corresponding adsorption characteristic lines. An online calculation, based on an adsorption and desorption model, can also be provided for the relevant emission control components.
In a further arrangement of the invention a fraction of an HC storage capacity of the at least one emission control component or an emission control component dominant with regard to a smoke-causing HC desorption is preset as HC storage amount limit value. The HC storage capacity as an HC amount that can maximally be stored is typically highly dependent on the temperature of a respective emission control component and additionally on its type and/or ageing state. The HC storage capacity is conveniently determined empirically beforehand for all emission control components relevant for an HC adsorption and is stored in a control device. An ageing dependence can thereby be considered in addition to a temperature dependence. For an SCR catalyst of the iron or zeolite type, the HC storage capacity is typically in the region of 1 g to 30 g per I catalyst volume at low temperatures of 0° C. and less. By means of the orientation of the HC storage amount limit value provided according to the invention as the HC storage capacity determined in such a manner, an undesired high load of the SCR catalyst or of another emission control component relevant with regard to this is avoided. It is thereby particularly advantageous if the fraction of the HC storage capacity is given in dependence on temperature, particularly decreasing with a decreasing temperature of the corresponding emission control component(s).
In a further arrangement of the invention, the cold start engine operation method is deactivated after reaching a predefined second threshold temperature for at least one of the emission control components or an emission control component dominant with regard of a smoke-causing HC desorption. The inventors have recognized that a storage of HC in zeolite-containing emission control components above a typically catalyst-specific threshold temperature is low or even negligible, wherein possibly stored HC can already desorb below this temperature almost completely. If the cold start engine operation method is deactivated as fast as possible after reaching the threshold temperature, excess fuel consumption is thereby also avoided or at least limited. If the engine load required by the user exceeds a predefined minimum value, where it is ensured that a further heating or at least no cooling takes place, the cold start engine operation method is preferably deactivated immediately after reaching the threshold temperature. In the other case, this can stay activated for a certain time. It is particularly advantageous in this connection to apply the exact time of the deactivation depending on the engine load. A sensor can also be provided to sense an HC desorption and to deactivate the activated cold start engine operation method by terminating some of all measures taken thereby after exceeding a detected HC desorption maximum.
In a further arrangement of the invention, a multiple injection of fuel into one or several cylinder combustion chambers of the internal combustion engine is carried out with an activated cold start engine operation method, which comprises a first pilot injection, a second pilot injection following the first pilot injection and a main injection following the second pilot injection within a work cycle of the respective cylinder. It can thereby be provided to divide the first and/or the second pilot injection into two individual injections following each other quickly. By means of the at least two pilot injections preceding the main injection, an ignition of the injected fuel is also enabled with low engine temperatures below the freezing point. Preferably, a comparatively low fuel amount of about 20% or less with regard to the main injection amount is injected in the first or in the second pilot injection. In this manner, an ignition is achieved even at very low surrounding or engine temperatures of minus 20° C. or less. Due to the low pilot injection amount, a temperature decrease caused by evaporation is at least reduced and an ignition of the homogenized pilot injection amount is improved.
It is particularly preferred if the first pilot injection takes place in a crankshaft angle region of larger than 20 degrees before an upper dead center in the compression cycle of the respective cylinder in a further arrangement of the invention. Typically, the temperature in the cylinder is too low for a conventional diffusion combustion at low temperatures of minus 20° C. or less. With the early pilot injection according to the invention, a homogenization of the mixture is enabled, whereby the ignitibility is improved. A combustion conversion of the first pilot injection takes place with a corresponding ignition delay, which leads to an increase of the temperature level in the cylinder. The fuel amount introduced with the second pilot injection can thus evaporate quickly and also ignite.
In a further arrangement of the invention, the second pilot injection takes place at a time after the start of a conversion of fuel injected by the first pilot injection. By means of the choice of the time for the second pilot injection according to the invention, the combustion progress for the fuel of the second pilot injection and the following main injection is improved.
A further improvement of the combustion progress is enabled, if the main injection takes place at a time after the start of a conversion releasing heat by fuel injected by the second pilot injection in a further arrangement of the invention. A safe ignition is thereby ensured even with very low temperatures. In this manner, HC emissions can also be kept comparatively low even with very low outer temperatures and a defined heating of the emission control system is enabled. The main injection typically only takes place behind the upper dead center of the compression cycle, particularly only after about 10 degrees crank angle behind the upper dead center. A late combustion position or a late position of the combustion center of mass thereby results. This enables a safe ignition and a defined and quick heating of the emission control system and thus the preferably provided zeolite-containing SCR catalyst. A NO formation caused by combustion is additionally reduced.
In a further arrangement of the invention, the implementation of the cold engine operation method takes place in a predefined low load region of the internal combustion engine and the cold engine operation is deactivated with an internal combustion engine load above the low load region. After the deactivation of the cold start engine operation method, a combustion method with a dominating diffusion combustion is preferably adjusted.